The present technology relates to a technical field for a speaker device, and more particularly, to a technical field for providing improved acoustic conversion efficiency and improved sound quality by inhibiting a magnetic fluid filled in a magnetic gap from flying off.
Some speaker devices have a ring-shaped magnet, a yoke having a center pole portion and a plate formed with a magnetic material. A voice coil wrapped around a coil bobbin is held in a magnetic gap formed between the center pole portion and plate. In such a speaker device, when a current is passed through the voice coil, the coil bobbin moves in the axial direction of the center pole portion, thus producing a sound.
Further, some of the above speaker devices have an elastic damper formed in a ring shape. The inner circumferential portion of the damper is connected to the outer circumferential surface of the coil bobbin, with the outer circumferential portion of the damper connected to the frame serving as an enclosure. The damper has the capability of holding the voice coil in a magnetic gap without the same coil touching the plate when the coil bobbin moves.
However, the damper accounts for a certain percentage of the total weight of the speaker device. Therefore, the speaker device is heavy because of the damper, thus inhibiting the movement of the coil bobbin and resulting in reduced acoustic conversion efficiency. The damper accounts, for example, for about 15% to 20% of the total weight of the speaker device.
For this reason, a magnetic fluid is filled in a given portion of some speaker devices rather than using a damper, thus reducing the weight of the speaker device and providing improved acoustic conversion efficiency (refer, for example, to Japanese Patent Laid-Open Nos. 1996-79886 (Patent Document 1) and 2003-32791 (Patent Document 2)).
In the speaker device described in Patent Document 1, a magnetic fluid is filled in a magnetic gap formed between the center pole portion and plate, and a voice coil wrapped around a coil bobbin is held in the same magnetic gap.
In the speaker device described in Patent Document 2, a shaft is attached to a center cap arranged on the tip side of the coil bobbin. The tip of the shaft is inserted into a through hole formed in the center pole portion via a bushing with a magnetic fluid filled between the shaft and bushing. The magnetic fluid is filled where the magnetic flux density is maximum in the center pole portion.